1. Field of the Invention
This invention relates to methods for hydroforming transition duct bodies used in gas turbines.
2. Description of the Related Art
Associated with gas turbines having multiple cannular combustors are transition ducts that carry hot gases from the combustors to the turbine inlet as shown schematically in FIG. 1. The combustors 12 are round, but the turbine inlet is annular. Therefore, the transition duct bodies 10 must have round inlets 16 and an exit 18 that forms a segment of an annulus.
The highly curved walls of the duct body 10 are difficult to fabricate. The difficulty is compounded by an offset 14 between the duct inlet 16 and duct exit 18. The offset 14 is the distance between the centerline of the combustor 12 and the centerline of the duct exit 18.
According to the current art, large transition pieces are fabricated by welding together a number of individual components. The largest component is the main body of the duct 10 shown in FIG. 2. It is typically made of two curved shells 20 and 22 that are stamped separately, trimmed to size, and then welded together. The welds 11 are shown in FIG. 1.
To facilitate removal from the dies after stamping of the two separate parts, the joints between these parts must pass through the widest contour lines on the sides of the duct body 10. Consequently, the longitudinal welds 11 terminate in the highly stressed upper corners of the duct exit 18 and have the effect of weakening these corners. This makes the longitudinal welds undesirable.
In addition, some duct bodies 10 require circumferential welds. Circumferential welds would be needed, for instance, to attach a frame for exit seals or support brackets, not shown in the drawing. They would cross the longitudinal welds in the duct bodies 10, thus producing more weak spots. Inherently, welding causes weld distortion. To achieve the required dimensional tolerances, special fixtures are typically required for welding, stress relieving, and heat treatment.
The current methods of fabrication are difficult and costly. Some large transition duct bodies cost more than a full-size automobile, each. A set of four to fourteen transition ducts per gas turbine represents a prime target for cost reduction.
Considerable progress was achieved by Yoshitomi et al, U.S. Pat. No. 5,735,156, which is not admitted to being prior art by its mention in this Background section. Yoshitomi et al use liquid pressure to make two transition pieces out of one work piece in one forming operation. The work piece could be a straight pipe or a combination of straight and conical pipes. To form a pressure vessel, the ends of the work piece are sealed with conical plungers thrust against the pipe ends by hydraulic cylinders. The pipe ends are held in the horizontally split, massive frame that encases the upper and lower dies which hydroform the work piece. The apparatus is complicated. It consists of split dies and a split frame that holds the work piece and requires five hydraulic cylinders to operate.
What is needed, therefore, is a less costly method for making stronger transition duct bodies that does not require longitudinal welding.